Technical Field
The invention relates to a sealed battery and a battery jacket can.
Related Art
In a sealed battery such as a cylindrical alkaline battery or a bobbin-type lithium battery, a battery jacket can serves as a collector of one of the electrodes. The battery jacket can accommodates the active parts inside thereof and its opening is sealed by a sealing part. In such a sealed battery, the battery jacket can constitutes the greatest portion of the total surface area of the battery, and is required to have high durability than the other components of the battery. Specifically, the battery jacket can is required to have corrosion resistance of preventing rusting during long-term storage or under environmental conditions at high temperature and high humidity. In a battery jacket can (hereinafter referred to as a cell can) for a sealed battery, materials containing Ni such as Ni-plated steel sheet or austenitic stainless steel have been employed to improve corrosion resistance.
Whereas a cell can has to have the foregoing corrosion resistance, the cell can has to have good sealing quality so that its opening is certainly sealed by a sealing plate and leakage of active parts which are accommodated inside is prevented for a long period of time. Methods for sealing a cell can include: crimping in which the diameter of the opening of the cell can is reduced and a sealing part is attached to the opening; laser welding; or the like. Materials of the cell can are described in Japanese Unexamined Patent Application Publication No. 2010-238462, etc., and the sealing method is described in Japanese Unexamined Patent Application Publication No. 7-37611, etc.
In conventional sealed batteries, a cell can is made of steel plate containing Ni. A cell can made of, for example, Ni-plated steel cannot achieve enough corrosion resistance. This is because water enters from pin holes, which are produced when a plate material is shaped into a cell can. Austenitic stainless steel does not have deposit, and therefore pin holes are not produced, in principle, in a cell can made of austenitic stainless steel. But, Ni-rich austenitic stainless steel, which is rare metal, is expensive. In such sealed batteries, improvement of the corrosion resistance of cell cans with cost as low as possible is in demand. Material of a cell can is required to have excellent corrosion resistance and to be inexpensive.
The material of a cell can will be discussed for sealing performance. Sealing performance is determined according to a sealing method as well as the material of a cell can, and it is necessary to choose a material suitable for the sealing method. As a sealing method, there are a method by the foregoing crimping and a method by laser welding. In the sealing method by crimping, external force exerted on a cell can during the processing has to be accurately adjusted according to mechanical properties of the material of a cell can (e.g. flexural strength); this is because the opening of the cell can is mechanically deformed and a sealing part fits to the opening. Electrolyte solution will be gradually reduced during long-term storage because of climbing movement of electrolyte (which is called as creeping).
On the other hand, in the sealing method by laser welding, since a sealing part and a cell can are joined to each other by welding, leakage due to the electrolyte creeping will not occur in principle after sealing. Sealing by laser welding is more appropriate in the light of reliability, including corrosion resistance. In the sealing method by laser welding, the electrolyte creeping will however occur at the time of welding by laser beam. Specifically, in a laser welding process, a sealing part is inserted into a cell-can opening before laser beam irradiates the sealing part, and the insertion causes the electrolyte creeping because the edge of the sealing part comes into close contact with the inner surface of the cell can. Thus, if it takes long time to finish sealing by laser welding, welding will be insufficient because electrolyte solution enters into the contact surface between the sealing part and the inner surface of the cell can. It is therefore necessary to finish laser welding for shorter time using high-power laser beam.
Because electrolyte creeping is less likely to occur in a cooled electrolyte solution whose viscosity and surface tension are low, Japanese Unexamined Patent Application Publication No. 7-37611 described the invention in which a cell can filled with cool electrolyte solution is sealed by laser welding. However, this requires a process for cooling electrolyte solution, which results in complexity of the manufacturing process. This also requires energy for cooling electrolyte solution. That is, the invention described in Japanese Unexamined Patent Application Publication No. 7-37611 will increase cost. It is, therefore, necessary for a cell can to be made of a material which can be processed by laser welding for shorter time without increasing the power of laser beam.